Diagnostic tests frequently require separation of a patient's whole blood sample into components, especially cellular portions from non-cellular portions e.g. serum or plasma from cells. For instance, plasma is obtained from anticoagulated blood and still contains all of the coagulation proteins, whereas serum is obtained from clotted blood with the proteins being retained with the clot and red blood cells. Samples of whole blood are typically collected by ventipuncture through a special cannula or needle attached to a syringe or an evacuated collection tube. The sample of blood in the form that is to be separated into components is typically drawn, using a needle, through a penetrable self-sealing elastomericclosure or other stopper into an evacuated tube. Separation is then accomplished e.g. by rotation of the tube in a conventional centrifuge e.g. a swinging bucket or a fixed angle centrifuge, as the different components of the whole blood have different densities, as described in U.S. Pat. No. 4,152,269 of A.L. Babson.
An apparatus and method of separating blood phases by rotation of a tube about its longitudinal axis i.e. axial rotation, are described in U.S. Pat. No. 4,828,716 of J.A. McEwen et al. The blood sample is introduced to the tube through a cap assembly that consists of a pierceable closure and a separator that has a one-way valve. The tube is then rotated about its longitudinal axis; the heavier cellular phase lines the tube wall and thereby separates from the lighter non-cellular (plasma or serum) phase. Once separation has been achieved, an axial probe penetrates the pierceable closure, detaches the separator from the closure and forces the separator down the tube. The axially-located non-cellular phase passes through the separator. An optical sensor is utilized to detect when the cellular phase begins passing into the separator, and to stop movement of the separator. Thus, the two phases are physically separated. However, it is believed that a separator that is more reliable in operation and which may be manufactured in a cost effective manner is required. A separator of different design that is believed to be effective in maintaining the phases in a physically separated form is disclosed in the copending application of G.A. Adams and R.P. Luoma, filed concurrently herewith.
In the separation of a liquid, especially blood, into phases, it is important that cross-contamination of samples be minimized and preferably avoided. In particular, whole blood should not contaminate serum fractions, as can occur in tubes conventionally used in the trade.